In such valves the external signal may be fluid pressure and the fluid pressure may control the opening and/or closing of the valve.
U.S. Pat. No. 5,829,473 describes a hydraulic or a pneumatic control signal that is introduced into a hydraulic valve by way of hydraulic or pneumatic pressure. This pressure applies a force on a diaphragm thus causing the diaphragm to move to a position in which it closes the valve for passage of fluid therethrough.